1. Field of the Invention
The present invention relates to antireflection films for coating the surfaces of optical elements and to optical elements having the same. In particular, the present invention relates to an antireflection film advantageously used for an ArF excimer laser beam having a wavelength of 193 nm.
2. Description of the Related Art
Fluoride films have generally been used as an antireflection film for vacuum ultraviolet light. For example, Japanese Patent Laid-Open Nos. 7-244205 and 7-244217 have disclosed optical elements using a fluoride film as an antireflection film. It is, however, difficult to reduce the reflectance of the fluoride antireflection film to 0.2% or less in a targeted wavelength band. Also, the fluoride film has an inferior environmental resistance.
Accordingly, oxide films having an environmental resistance superior to that of the fluoride film, such as Al2O3 and SiO2, are being increasingly used. Oxide antireflection films have been disclosed in Japanese Patent Laid-Open Nos. 09-258006 and 2000-206304. These antireflection films are composed of high-refractive-index layers containing Al2O3 and low-refractive-index layers containing SiO2 that are alternately layered.
As shown in these publications, each high-refractive-index layer and each low-refractive-index layer are paired and layered on the optical element. The performance of an antireflection film depends on the number and thickness of the layers. In general, as the number of layers and the number of interfaces between the high-refractive-index layers and the low-refractive-index layers increase, the antireflection characteristics are enhanced. Also, the above publications disclose that the enhanced antireflection characteristics lead to an optical element having an increased transmittance. Therefore, antireflection films are generally formed to at least a specific thickness.
Films always absorb a certain amount of light when they transmit the light. If the geometrical thickness of the antireflection film increases, the absorption also increases, and, consequently, the transmittance of the resulting optical element is reduced. In general, as the refractive index of a film increases, the film absorbs more light. Accordingly, absorption is hardly taken into account in the low-refractive-index layers, such as SiO2 layers. In the high-refractive-index layers, such as Al2O3 layers, however, absorption is not negligible from the viewpoint of optical characteristics. If an antireflection film is produced taking only reflection into account, without considering the absorption of ultraviolet light by the Al2O3 layers, the transmittance of the resulting antireflection film is reduced. Thus, the resulting element cannot sufficiently function as an optical element. The foregoing Japanese Patent Laid-Open Nos. 09-258006 and 2000-206304 have not described the absorption of ultraviolet light in the antireflection film.
In general, light absorption is expressed with an extinction coefficient. FIG. 12 is a plot showing the relationship between the refractive index and extinction coefficient of an Al2O3 layer for light of 193 nm. The refractive index is changed by varying the amount of fluoride gas introduced for forming the film. The horizontal axis and the vertical axis in FIG. 12 represent the refractive index and the extinction coefficient, respectively. FIG. 12 suggests that if the refractive index of a film is 1.67 or less, the extinction coefficient can be assumed to be 0. In a film having a refractive index of 1.67 or more, however, the extinction coefficient is more than 0, and it increases as the refractive index increases. The extinction coefficient will be described later in detail.